OBJECTIVES: To identify the etiology and pregnancy outcome of hydrops fetalis in a cohort of pregnancies referred to a tertiary maternal fetal medicine center in the UK. These data allow the review of a large series of pregnancies affected by hydrops fetalis and emphasize the importance of investigation and then treatment of individual cases. This provides parents with improved information and especially specific prognostic information. METHODS: A retrospective review of 63 consecutive cases of hydrops fetalis managed between September 1996 and March 1999. RESULTS: Of the pregnancies, 12.7% (n = 8) were associated with an 'immune' etiology. Of these, 62.5% (n = 5) had fetal anemia due to anti-D, 25% (n = 2) anti-Kell and 12.5% (n = 1) anti-c antibodies. The remaining 55 cases (87.3%) had a non-immune cause. Eight (14.5%) were due to human parvovirus B19 infection. Fourteen cases (25.5%) were associated with aneuploidy and, in four (7.3%), a primary hydrothorax was the cause of the non-immune hydrops fetalis. A cardiac cause was found in five (9.1%) cases. Three of these had supraventricular tachycardia and one had congenital complete heart block. Cystic hygroma was associated with hydrops fetalis in six cases. Twin-twin transfusion syndrome was the cause for hydrops in two cases. Massive transplacental hemorrhage was identified in one case. Fetal akinesia and muscular dystrophy caused hydrops in one case each. In 14.5% (8/55) of cases no obvious cause was identified and these were classified as 'idiopathic'. Three other cases could not be classified because parents declined investigations (unclassified). In the pregnancies with non-immune hydrops fetalis, the outcome was favorable in 27.3% (15/55) of cases. CONCLUSION: The prognosis of hydrops fetalis differs markedly between different etiological groups. Etiologies range from treatable causes with a good outcome and probably no long-term side-effects (as in case of parvovirus B19), to others which are incompatible with life or are associated with considerable perinatal morbidity and mortality.